Heyo, we'll be building an assassin Rengar. The key is to do so much damage that they can't even react, even with the "!" above their head! This build snowballs very quickly and after so many games with Rengar, I think this, in my opinion, is the best build. Rengar is a cat that specializes in Assassinating anyone, you just stab stab stab and they're dead. Make use of bushes to maneuver around, and quickly kill your target!

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This is the only one for jungle. Nothing else.
Flash is better than ghost in every Rengar scenario, stop it!

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Personally, I think that Flash + Ignite is the strongest combo. Mainly because the much faster paced game that league is now. Rengar has a kit that is very easy to snowball out of control. You'll need to shut him off earlier for any counter play. You can also take Flash + TP for the later game, or Flash + Exhaust. Any could work pretty decently, but Ignite for that early snowball so you can begin murdering everyone.

JUNGLE is always Machete + 3 potions. I do not like the hunter's potion on Rengar at all. Next up rush the jungle warrior item. It's too good to pass up, 60 ad too strong. Ionian boots to help finish your 40% CDR! Then I like Tiamat next for that insane burst that comes with Empowered Q and it's active. Get Youmuu's because it's also too good to pass up. Then Finish your Hydra for all it's stats. Rapid Fire or Shiv next. I like Rapid Fire PERSONALLY. But Shiv is great too, it's more combat stats so it'll actually be stronger, however Rapid Fire 8% movement speed... Lastly get Infinity Edge to be a crit MONSTER. 1 shot squishies and cut through tanks like butter.

My FINAL Jungle build looks like this:

For TOP LANE, I like starting long sword personally, but Doran's Blade is pretty good. Doran shield only if you are in 2v1 or
scared. Rush Tiamat for that active and q combo, with mobies for insane mobility/ roam potential. Youmuu's because its just AMAZING on Rengar, there's no reason not to get it, CDR/AS/AD/MS. Finish Hydra for the Life steal/AD/ Burst. Rapid fire can because crit/attack speed bonus is insane. Infinity Edge for the DPS. And close out with Black Cleaver so TANKS can't even 1v1 you.

My FINAL Top Lane build looks like this

His passive is the best passive in the game IMO. Unlimited jumping in a bush! When top lane, CS from the bushes safely. You pretty always get the first auto so use that to your advantage. And use your E as you jump to pretty much guarantee it landing (it's pretty difficult to land TBH).

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R>Q>E>W
Max Ultimate first when you can, it's too useful not to. Pretty much the entire reason this build works.
Max Q next. It's your main damage.
Max E next. It's got great utility, and especially when Roaming in the later points of the game, the slow is crazy.
Max W last. The heal scales with level, damage scales with points. So you'll mainly be using this for the sustain, and clutch heals during a fight.

I ALWAYS GET W LEVEL 2 UNLESS IM GETTING/DOING AN EARLY GANK. Heals just 2 OP.
SOMETIMES YOU CAN MAX E FIRST, only in 1v2's. It helps with CSing if you dont think you can get roaming done.

Thunderlord's OP. That's it. If you manage to JUMP W, E, You get the proc. It's broken right now. So I can't really recommend anything else. I mean stormraider's surge is okay, but Thunderlords is just so good. Everything else is just self explanatory. You could get wanderer if you wanted to. Don't get sorcery in the ferocity tree.

The runes I run in the jungle gives 40% cdr. Jungle Item + Boots + Youmuus + 10% will give 40% cdr! The runes I run top will also give you 40% cdr! BC + YOUMUUS + 10% will give you 40%! In top lane, you could not take the runes and just normal MR and get the same result with Cdr Boots, but mobies are so important! 40% cdr gives a 42 sec ult so basically you'll be using it all the time.

Okay. Jungling is jungling. It's pretty easy to jungle on Rengar on the new patch. Start out with Machete and 3 Potions, just start q, and stack up. I like the use of Empowered W to clear mostly, but do note that Empowered Q does make your jungle clear faster. I like the sustain that W gives, but it wouldn't be wrong to use your Empowered Q. Personally, I always start from the bottom side, but you can start Top side if you plan to level 2 invade or something. His early game isn't god awful, and you can stay relatively healthy in the jungle. Ganking is super easy. STACKING TO 5 MAKES IT SO MUCH EASIER. You'll definitely need to pick up/ master how to do the EMPOWERED E and Jump combo, where you jump and use your bola at the same time for a guaranteed hit. The bola is your only form of CC so LAND IT! You dish out ton's of damage so you'll win most fights. You can lane gank for bot and top, just make sure the bottom bush/ top bushes aren't warded!

Rengar is pretty easy to CS on. Early on from 1-2 just focus on get your ferocity up to level 5. 1-2 is critical for you to gain an early lead. Jump and follow with Q, just jump with Q in hand so you can run immediately. Avoid taking minion aggro! Get E level 2 if you're getting an extremely early gank, but most times just get W. W can push the wave really quickly! And keep in mind W is RANGED! So use it at a distance unless all-inning. At level 3, if they aren't a tank, and you've managed to poke them low enough with bush jump -> q,and YOU HAVE 5 FEROCITY then just go in the bush, wait for a moment till minions are clear and proceed to maul them. REMEMBER THAT YOU NEED YOUR COOLDOWNS FOR DOUBLE Q! Use ignite mid-fight to get the full damage! You pretty much win every fight because Empowered Q and Normal Q are too much damage for any squishy to take.

PROS:

Super High DPS
INSANE Burst Potential
Perhaps one of the most fun passives in the game
Much room for outplay
Broken
Low CD Ult
Snowballs easily
Great against INVIS champs

CONS:

Very kill reliant
His ult is basically the only gap closer if you're not near a bush
Easily kited without a bush
Pink wards destroy his purpose
Invulnerability items and spells ruin him

To be honest, Rengar sucks at Team Fighting a lot. Specializes in picks. Just kill people who are alone, if you don't see anyone just use your ult to find picks. PICK! Grouping is bad news for the Cat. If the enemy team groups, split them up! It's really important to make picks. Spam your ultimate! Use it when you're going to hunt. For early game, get some deep wards in the enemy jungle, and as soon as you get 6 and see them, go ahead and attack! As you get closer to mid game, and you build more cdr, use your ult to make easy picks on people playing too aggressive. At the late game, you should be near 40% cdr, and you SPAM THAT ULTIMATE! Even if you don't see anyone! Never, ever, ever forget that your ultimate reveals champions, and don't be afraid to use it for just vision! Make sure to tell your team you don't have ult if you think there will be a team fight. Without bushes, you tend to get kited to death.